Since the announcement by Kodak of multilayer structure organic EL devices using a vacuum evaporation technique, the development of organic EL displays has been a great concern and reached the verge of commercial application.
In organic EL devices, what becomes a problem in injecting electrons from a metallic cathode into an organic compound which is deemed to be substantially insulating is the energy barrier. It is desired to reduce the energy barrier.
From the standpoint of reducing the injection barrier, metals having a low work function and salts or oxides thereof are currently used as the cathode or electron injecting electrode.
For the fabrication of multilayer structure organic EL devices, vacuum evaporation of low molecular weight dyes is a common practice. The vacuum evaporation technique, however, is not regarded efficient as the device fabrication process because it is difficult to form homogeneous, defect-free thin films and time consuming in depositing a plurality of organic layers.
JP-A 4-337284, JP-A 11-54270 and JP-A 11-40358 disclose organic EL devices wherein organic layers are formed by a coating technique which is generally believed efficient in productivity. However, these patents use organic layers of the single layer structure, and do not take into account organic layers of the multilayer structure capable of more efficient light emission.
JP-A 4-2096 and JP-A 2000-77185 disclose organic EL devices wherein organic layers of the multilayer structure are formed by coating. In Examples of JP-A 4-2096, a hole injecting and transporting layer is formed by coating a polymeric hole injecting and transporting material, or a hole injecting and transporting material and a polymeric binder, and a light emitting layer is formed thereon by coating a light emitting material and a polymeric binder. In JP-A 2000-77185, an organic polymer having a siloxane structure is contained in organic layers to be laminated. When organic layers of multilayer structure are formed, at least a lower side organic layer must contain the organic polymer having a siloxane structure. According to its disclosure, organic polymers having a siloxane structure are contained in organic layers to be laminated, whereupon the coatings are crosslinked and insolubilized to unite the polymers together to join the layers; or a coating solution containing an organic polymer having a siloxane structure and another organic polymer having a carbon base structure and a less compatibility therewith is coated, whereupon the polymers are laminated by utilizing two phase separation within the coated layers. In Examples, respective organic layers to be laminated are formed by coating solutions of respective functional compounds and a silicone resin in a common solvent (typically, tetrahydrofuran THF), followed by crosslinking; and respective organic layers are formed by using an organic polymer having a carbon base structure and causing two phase separation. In these multilayer structures, however, particularly when a common coating solvent is used, the construction of organic layers which can be formed as a laminate is restricted, and the compound which can be used in introducing a crosslinking structure is limited. The functional groups which must be introduced into the light emitting layer for crosslinking serve to reduce the efficiency of light emission of the resulting organic EL devices.
Polymeric organic EL devices which are now generally used often rely on the structure that a Ca cathode is formed on a light emitting polymer by evaporation. In such electrodes, the electrode material can diffuse into the polymer to become a dopant or to quench luminescence, resulting in devices which are often short-lived in continuous driving. Also, to prevent the Ca cathode from oxidation, the conditions of sealing after the cathode evaporation must be strictly managed.
Such a situation prevails not only with the Ca cathode, but also often prevails with many cathodes known as electron injecting electrode. Then a cathode or auxiliary electrode of an ordinary wiring material, typically Al is often used in combination with the electron injecting electrode.
There are known some improvements in such cathodes. For the purpose of using as the cathode material a stable metal which has been commonly used as the conventional wiring conductor, it is proposed to construct an organic layer in close contact with the cathode electrode so as to provide a lower energy barrier to electron injection (see JP-A 10-270171, JP-A 10-270172, JP-A 11-233262 and JP-A 2000-182774).
Illustratively, JP-A 10-270171 discloses the provision of an organic compound layer doped with a metal which functions as a donor (electron donative) dopant, at the interface with the cathode electrode. More specifically, on a light emitting layer resulting from evaporation of tris(8-quinolinolato)aluminum complex, a layer of tris(8-quinolinolato)aluminum complex doped with Li or Mg, or a layer of bathophenanthroline doped with Li is formed by evaporation. Alternatively, poly(p-phenylene vinylene) is deposited as a light emitting layer by the method of Burroughes et al., a polystyrene film containing anthracene/Li is then spin coated in a nitrogen atmosphere, which serves as a metal doping layer, and a cathode electrode is formed thereon by evaporation of Al.
JP-A 10-270172 teaches the provision of an organic compound layer doped with a metal oxide or metal salt at the interface with a cathode electrode. More specifically, on a light emitting layer resulting from evaporation of tris(8-quinolinolato)aluminum complex, a layer of tris(8-quinolinolato)aluminum complex doped with LiF or Li2O or a layer of bathophenanthroline doped with Li2O is formed, which serves as a doping layer, and a cathode electrode is formed thereon by evaporation of Al.
JP-A 11-233262 discloses that an organic layer in close contact with a cathode electrode is constructed from an organic metal complex containing at least one alkali metal ion, alkaline earth metal ion or rare earth metal ion and that the cathode electrode is constructed from a metal capable of reducing the metal ion in the organic metal complex into a metal in vacuum. More specifically, on a light emitting layer resulting from evaporation of tris(8-quinolinolato)aluminum complex, an organic layer (or electron injecting layer) of mono(8-quinolinolato)lithium complex, mono(8-quinolinolato)sodium complex, mono(2,2,6,6-tetramethyl-3,5-heptanedionato)lithium complex, mono(2,2,6,6-tetramethyl-3,5-heptanedionato)sodium complex, di(2,2,6,6-tetramethyl-3,5-heptanedionato)magnesium complex, di(2,2,6,6-tetramethyl-3,5-heptanedionato)calcium complex or tri(1,3-phenyl-1,3-propanedionato)-mono(bathophenanthroline)europium complex is formed by evaporation, and a cathode electrode is formed thereon by evaporation of Al.
JP-A 2000-182774 discloses that a mixed layer comprising an organic metal complex containing at least one alkali metal ion, alkaline earth metal ion or rare earth metal ion and an electron transporting organic compound is formed as an organic layer in close contact with a cathode electrode, and that the cathode electrode is constructed from a metal capable of reducing the metal ion in the organic metal complex in the mixed layer into a metal in vacuum. More specifically, on a light emitting layer resulting from evaporation of tris(8-quinolinolato)aluminum complex, a mixed layer of tris(8-quinolinolato)aluminum complex and mono(8-quinolinolato)lithium complex is formed by evaporation, and a cathode electrode is formed on the mixed layer by evaporation of Al. The materials of the cathode electrode are limited to high-melting metals.
However, these patents essentially correspond to devices of the type wherein organic layers are formed by evaporation. Heretofore, no study has been made on the technique required to form a multilayer structure by a coating method nor on the electron injecting layer which is optimum to enhance the efficiency of polymeric light emitting material. JP-A 10-270171 describes the coating of a metal doping layer on a polymeric light emitting layer while the formation of the polymeric light emitting layer is conducted by the method of Burroughes et al. The method of Burroughes et al. involves coating a precursor of poly(p-phenylene vinylene), often abbreviated as PPV, and heating it for deacidification, resulting in PPV. The PPV is a polymer which is insoluble in any solvents and allows an overlying layer to be readily coated thereon, helping form a multilayer structure. On the other hand, the finished PPV raises a problem with respect to uniformity and has the drawback that device characteristics are very poor as compared with the recently developed soluble PPV. Since metallic Li having a considerably high reactivity is admixed, control of the atmosphere is very difficult. To coat a low molecular weight anthracene which is likely to crystallize, it must be dispersed in a non-conductive polystyrene polymer.
For the above reason or other, when an organic layer is formed by a coating method, it is desired to have a technique of using an inexpensive and stable cathode material and ensuring a fully high electron injection efficiency. In the case of coating which entails a combination of a solvent with a material, the material suited for the evaporation is not directly applicable.
When a metal having a low work function is used as the cathode, it is very unstable independent of the way of forming the organic layer. Attempts have been made to stabilize the cathode metal. Such a metal is used as an alloy as disclosed in, for example, JP-A 5-198380.
However, it is still necessary to use metals having a low work function and salts thereof, which are cumbersome to handle during the manufacture. Degradation of such metals is still a problem.